I use a Brompton daily for commuting. I've been using the fixed longer seatpost for the last year or so. Even so seat is lower than ideal. But then it's a folding bike! My tailor would say I have a 32" leg.

I've also started riding a road bike for triathlons. (Middle aged crisis etc) And I'm getting pain in a knee joint. It seems to be worse after riding the Brompton.

Now I reckon I'm riding the Brompton faster that I was and the lower than ideal seat height is to blame. Frequent starts with limited gears presumably don't help. What does the forum think?

Obviously I should get the telescopic pillar. But concerned it's more complication. And greater stress on the frame.

I use a Brompton daily for commuting. I've been using the fixed longer seatpost for the last year or so. Even so seat is lower than ideal. But then it's a folding bike! My tailor would say I have a 32" leg.

I've also started riding a road bike for triathlons. (Middle aged crisis etc) And I'm getting pain in a knee joint. It seems to be worse after riding the Brompton.

Now I reckon I'm riding the Brompton faster that I was and the lower than ideal seat height is to blame. Frequent starts with limited gears presumably don't help. What does the forum think?

Obviously I should get the telescopic pillar. But concerned it's more complication. And greater stress on the frame.

I suffered that same problem for years. The first thing you need to do is adjust your saddle height properly. While sitting on your saddle, place the center part of your foot on the pedal. Your leg should be fully extended -- comfortably, not stretched. If you can't set that, you will need to get a longer seat post. The Brompton has two lengths available. Secondly I would suggest using a supplement -- "Glucosamine & Chondroitin" combination. I suffered severely with knee pain in the right knee. Following a proper height setup with the saddle and taking that supplement work wonders for my knee. I have no problems at all anymore. Finally, while taking the supplement, don't over work your knee until it starts to feel better.

... Obviously I should get the telescopic pillar. But concerned it's more complication. And greater stress on the frame.

The Brompton telescopic seatpost will provide you with 110mm more extension, and it's not nearly as complicated as your knees. Also, if your saddle rails are in the lower position on the Pentaclip that attaches the saddle to the seat post, you should be able to get another inch by moving the saddle rails to the upper position on the Pentaclip.

-HANK RYAN-
Norman, Oklahoma USA

__________________
DISCLOSURE: I have an ownership interest in a company that is an authorized dealer for Brompton.

Trick is to pull the seatpost up high enough, every time you un fold the bike.

I too have the basic+6cm post, [similar leg length], once I find the better saddle height,

I use the recall of where the saddle edge touches the outside of my hip.

and pull it up till it touches there again.
...

I'm always using the seat post fully extended.
The problem seems to be caused by riding the Brompton rather than running or swimming - or the road bike. I tried a week on the tube. Knee improved - but miserable way to travel.
Another possibility could be knee deflection. I use the standard cage pedals - no clips and I've caught my left (problem) knee deflecting right on occasion. Now that I'm aware of it I'm keeping an eye on it - while maintaining a look-out for traffic of course.
Also, I use my left leg on the ground at lights (tried using the right but too confusing). I'm now trying to slide bum off seat and stand on both legs while waiting at lights for any time.

But I will order the telescopic seat post. A change may be as good as a rest after all.

I need to order a new set of cables, more pads and yet another rear rim. Third in three years.

Your brompton position is likely to be highlighting an existing symptom. When the glass overflows with symptoms you get a problem.

Ie don't assume its the bike, it will also a combination of your sleeping posture, walking gate and so forth.
Altering the bike might contain the symptoms, which is what you want to achieve, but be prepared to look more broadly for solutions.

Result!!!
New seat post and no knee pain. Only had it a week but looks good.
It's actually rather fun. Without the top bit extended it's the same length as a standard seat post so quite rideable for short distances. So you can unfold and hop on just as quickly if pressed. But extended to 'racing' height it feels really whizzy. Not so good at traffic lights though.
Thanks all.